Sphere Gap Breakdown Voltage Tables

The following two tables give the breakdown voltages for a standard sphere gap of the specified sphere size and spacing. The standard accuracy is ±3% for gaps less than half the sphere diameter and 5% for gaps larger than that. The first table is for a gap with one terminal grounded, typically a vertical gap. There are two columns for each sphere size depending on whether it is a positive or negative impulse. The second table is for a gap with both terminals at HV. The values from both of these tables will need to be corrected for current atmospheric conditions, particularly humidity. More details on sphere gap construction and procedures is available.

Sphere gap with one terminal grounded

A -> AC, DC, either polarity and full negative standard impulse voltages (one sphere grounded)
B -> positive polarity standard impulse voltages and impulse voltages with long tails (either polarity)

As the field gets more uniform (i.e. the gap is a smaller fraction of the sphere diameter), the difference in voltage for the two polarities becomes less. In general, voltages for spheres larger than the largest for which there is a voltage will be the same. That is, 200 cm spheres with a 0.5 cm gap will breakdown at the same voltage as a 15 cm sphere at the same spacing.

	Sphere Diameter (cm)
Gap Spacing	5		10		15		25		50		100		150		200
cm	A	B	A	B	A	B	A	B	A	B	A	B	A	B	A	B
0.5	17.4		16.9	16.8	16.9
1.0	32.0		31.7		31.4		31.2	31.4
1.5	44.7	45.5	44.7	45.1	44.7	45.1	44.7
2.0	57.5	58.0	58.0		58.0		58.0
2.5			71.5		71.5		71.5		71.5
3.0			85		85		85		85
3.5			95.5	96	97		97		97
4.0			106	108	108	110	110		110
5.0			123	127	127	132	135	136	136
7.5					181	187	195	196	199	199
10.0							257	268	259	259	262		262		262
12.5							277	294	315	317
15.0							309	331	367	974	383	384	384		384
17.5							336	362	413	425
20.0									452	472	500		500		500
25.0									520	545	605	610
30.0									575	610	700	715	730	735	735	740
35.0									725	755	785	800
40.0											862	885	940	950	960	965
45.0											925	965
50.0											1000	1020	1110	1130	1160	1170
75.0											1210	1260	1420	1460	1510	1590
100.0															1870	1900

Symmetrical sphere gaps

This data applies to gaps where neither sphere is grounded, or, where the gap is so far from the surroundings that the fields relative to ground have no influence.

All voltages in kiloVolts with air at 20deg C, 760 torr, either polarity impulse, AC, DC
spacings <0.5D Accuracy +/- 3% ; spacings >=0.5D accuracy +/- 5% (shown in italics)

	Sphere Diameter (cm)
gap (cm)	5	10	15	25	50	100	150	200
0.5	17.5	16.9	16.5
1	32.2	31.6	31.3	31
1.5	46.1	45.8	45.5	45
2	58.3	59.3	59.2	59
2.5	69.4	72.4	72.9	73
3	79.3	84.9	85.8	86
4		107	111	113	112
5		128	134	138	138	137	137	137
8		177	194	207	214
10				248	263	266	267	267
12				286	309
14				320	353
16				352	394
18					452
20					495	504	511	511
25					558	613	628	632
30						744	741	746
35						812	848	860
40						902	950	972
50						1070	1140	1180
60						1210	1320	1380
70							1490	1560
80							1640	1730
90								1900
100								2050